Copy&Paste to new workbook (English/Excel 2000)

I have a macro installed on about 20 office P.C.s that copies information from a working file on their local drive to a "gathering" file on a network drive (using Access for this project is not an option). The macro works well with one odd quirk. Each week the information from one user does not get transferred. It is a different user each week and none of them received any message boxes or other warnings when performing the transfer. I did test the macro when the network file was open and I received the message box stating that the file was unavailable for editing. I've included the portion of the macro that performs the paste operations and am looking for any ideas on why this is happening. THANKS 'opens Worked Permits worksheet
ChDir "P:"
Workbooks.Open Filename:="PataWorkedFile.xls"
'selects cell Al
Range("A1").Select
'moves to next available row
Selection.End(xlDown).Select
ActiveCell.Offset(1, 0).Range("A1").Select
Selection.PasteSpecial Paste:=xlValues, Operation:=xlNone, _
SkipBlanks:=False, Transpose:=False